Prior to certain further processing steps, e.g. prior to the etherification, cellulose must be activated. Depending on the processing conditions it may happen that no homogeneous activation of the cellulose takes place, because the crystalline regions are poorly accessible to the activating agents that are used. It is known to activate cellulose by swelling it with liquid ammonia (see H. A. Krassig, Cellulose Structure, Accessibility and Reactivity, Gordon and Breach Science Publishers, 1992). The ammonia molecule, because of its free electron pair on the nitrogen atom, can compete with hydroxyl groups of adjoining molecule chains and substitute OH--O--hydrogen bridges by OH--N bonds. This leads to an incorporation of ammonia in the crystal lattice and causes an expanding of the crystal lattice. Ammonia-cellulose complexes are formed. The formed ammonia-cellulose complexes are relatively unstable. When evaporating the ammonia, the triple growth in fibre diameter returns to its original dimensions. The complex is also destroyed when the ammonia is washed out with water or alcohol. Also here the original cellulose crystal structure is regularly regained.
During the subsequent derivation reactions a residual ammonia content is generally disturbing. During the acylation, for example, an undesirable ammonium salt occurs. This adversely affects the efficacy of the catalysts used in the process.
The DE 43 29 937 proposes that, in order to maintain the activation state after the ammonia swelling, under the effect of superheated steam the required residual ammonia should be replaced by water as swelling or inclusion agent. It has been found that the activation state is lost quickly when the material treated in this manner is not immediately processed further. According to the DE 0 108 991 the cellulose, after the swelling in liquid ammonia at a low temperature, is not recovered in the dry form but the mixture is mixed with an aqueous alkali hydroxide solution and the ammonia is removed in the presence of the alkalising agent. The obtained alkaline cellulose suspension is immediately subjected to an etherification reaction.
It is, therefore, among others an object of the process according to the invention described hereinbelow to make available a process in which the residual ammonia can be removed without problem while to a large extent maintaining the activation, without additional inclusion agents, e.g. water being required.
The U.S. Pat. No. 5,322,524 describes cellulose fibres or cellulose-containing fibres with an improved resistance to abrasion and an increased permeability with respect to chemicals. The increased permeability leads to an improved activity in respect of chemicals. According to the known proposal, cellulose fibres are treated in ammonia vapour, between room temperature and 140.degree. C. at about 7 bar (100 psi) to 120 bar (1,700 psi), for a sufficiently long time to change the inter-atomic planar distances in the cellulose and to obtain another modification of the cellulose in the form of stable crystalline cellulose III. The operation is carried out, for example, in a Parr cylinder and the pressure is reduced by opening same. Only ammonia escapes. Cellulose fibres remain behind in the Parr cylinder. The obtained fibres of crystalline cellulose III can be treated in ethylene diamine and can subsequently be subjected to boiling in dimethyl formamide to convert the cellulose III to cellulose IV. The stability of the cellulose III is proved by the fact that after one hour of boiling in water it cannot be converted to cellulose I. It is characterised by a specific X-ray diffraction spectrum with peaks at diffraction angles 2.theta. of 11.5, 15.5 and 20.5. The state of the art described in the foregoing corresponds to a large extent to that indicated in the publication "Textile Research Journal", July 1986, p. 419-424.
Also the publication "The Journal of Physical Chemistry", Vol. 41, No. 6, p. 777 to 786 only describes the treatment of individual fibres in liquid ammonia at -75.degree. C. Here a swelling of the fibres takes place due to the action of the liquid ammonia. After removing the fibres the ammonia is immediately evaporated unless a thin protective layer of low-boiling paraffin oil is used. The X-ray diffraction spectrum of the obtained fibres does not display a complete mutuality with the spectrum which characterises the cellulose activated according to the invention described hereinbelow.
The EP-A-0 077 287 relates to the use of liquid ammonia for activating cellulose contained in animal feeds, using high pressure. According to this the material that contains the cellulose is treated with liquid ammonia at high pressure. Then a quick reduction of the pressure to atmospheric pressure takes place, which causes boiling of the ammonia and a separation into fibres of the cellulose material. The cellulose starting material remains in the system, the internal pressure of which is reduced to atmospheric pressure.
The U.S. Pat. No. 3,707,436 proposes a process for producing cellulose for the paper manufacture, with which the lignocellulose material is impregnated with anhydrous ammonia in a closed chamber under pressure and the pressure is suddenly reduced, during which an explosion-like removal of the ammonia and a flinging out of the material takes place. The starting material preferably consists of wood chips which, in addition to cellulose, contain considerable quantities of lignin, hemicellulose and up to 100% water content. The wood chips are impregnated with so much ammonia that the system contains at least the same quantity of ammonia as water, and are heated to a temperature which suffices for their plastification. In the examples a mass ratio of ammonia to water in the range of 2 to 4 is used. The hemicelluloses remain behind in the product in the water-insoluble form. They make the material plastic and give strength to the paper products made from same. The obtained cellulose is slightly more amorphous and plastic than in the initial state.
The U.S. Pat. No. 5,171,592 describes a process for treating biomasses. The process comprises a.o. the "Ammonia Freeze Explosion" (AFEX), with which the biomass is swollen with liquid ammonia and is then exploded into a flash tank by opening a valve. During the explosion about 25% of the used ammonia are evaporated. Preferably, the biomass is predried by treating it with superheated ammonia vapour, deaerated and pre-heated. After the AFEX treatment the remaining liquid ammonia is expelled by a treatment with ammonia vapour.
In wood and other biomasses cellulose is present accompanied by lignin and hemicelluloses. Biomass furthermore usually contains at least 50% moisture. Lignin is a complex high-polymer natural substance which is embedded in the inter-fibrillary capillary spaces of the cellulose fibres. The fibrillary cellulose chains are joined by a thin cross-linked layer of lignin and hemicellulose to other fibrils to form a fibre bundle. The matrix of lignin and hemicellulose surrounds and protects the cellulose fibrils and holds the structure together, similar to a resin in a composite glassfibre material. From what has been mentioned it will be clear that during the treatment with liquid ammonia the cellulose in this rigid bond--except in the amorphous bonding regions--is hardly accessible to a swelling. The processes based on lignocellulose material are aimed more at forming a fibrous material from separate, but undamaged lignocellulose fibre cores. An activation of the cellulose contained in same hardly takes place. When the treated material is subsequently subjected to a separation of lignin and/or hemicellulose, during the extraction steps that are used this slight activation of the cellulose will anyway be lost. Processes which relate to the activation of lignocellulose material in the form of wood or another biomass do not, therefore, display any significant correspondence with the subject of the present invention.